1. Field of the Invention
The present invention relates to a driving device and driving method for driving a load such as a piezoelectric speaker as a capacitive device.
2. Description of the Related Art
A piezoelectric speaker mounted in an information apparatus, such as a mobile telephone, is characterized by being thin, light, and low power consumption. If sound quality is improved and price is lowered, the piezoelectric speaker may be popularized. However, in the present conditions, a capacitive device such as a piezoelectric speaker is a capacitive load for a driving circuit, and there is no optimal driving method for driving a capacitive device.
In contrast, an inductive device, such as a dynamic speaker, is generally used as an inductive load in a driving circuit. The driving circuit is configured to be suitable for the inductive load. In particular, when a driving circuit is incorporated in a mobile device, the inductive device is generally driven by a battery. A switching amplifier (class D amplifier) having high electric-power efficiency is suitable for long time use.
FIGS. 19 and 20 are circuit diagrams showing a load-driving state of a conventional switching amplifier. FIG. 19 shows load-driving states 3-aand FIG. 20 shows load-driving states 3-b. 
FIG. 21 is a waveform diagram shows respective signals according to the driving circuit of FIGS. 19 and 20 in the case that the duty ratio of a pulse modulation output is 50% . The characters in FIG. 21 correspond to the characters'respective positions in FIGS. 19 and 20.
The load-driving state 3-a in FIG. 19 is performed during duration T4a. Transistors 20 and 23 are turned ON and transistors 21 and 22 are turned OFF. The value at OUTP becomes VCC-level and the value at OUTN becomes GND-level. The potential difference VL1 across an inductive load L1 becomes +VCC. Then, a current I flows through the inductive load L1 from OUTP to OUTN.
The load-driving state 3-b in FIG. 20 is performed during duration T4b. Transistors 20 and 23 are turned OFF and transistors 21 and 22 are turned ON. The value of OUTP becomes GND-level and the value of OUTN becomes VCC-level. The potential difference VL1 across the inductive load L1 becomes −VCC. Then, the current I flows through the inductive load L1 from OUTN to OUTP.
In the above driving method, the duty ratio of the pulse modulation output becomes 50% at the time of a no-input-signal period that no electric power needs to be supplied to the inductive load L1. Then, the ripple of the current I that flows through the inductive load L1 becomes highest, thus largely producing wasteful current.
FIGS. 22 to 27 show improved examples of a switching amplifier having a driving method in which wasteful current consumption is reduced (refer to U.S. Pat. Nos. 6,614,297, 6,211,728, and 6,262,632).
FIGS. 22 to 27 are circuit diagrams and waveforms showing load-driving states of the switching amplifier.
As shown in the signal waveforms of FIGS. 26 and 27, in such a switching amplifier, a voltage across load terminals is applied during the duration that electric power is required, but connection of load terminals is short-circuited so as to make the applied voltage to be zero volts during the duration that no electric power is required, thus controlling an output waveform.
A load-driving state 5-a shown in FIG. 22, a load-driving state 5-b shown in FIG. 23, a load-driving state 5-c shown in FIG. 24, and a load-driving state 5-d shown in FIG. 25 are configured so as to reduce a ripple current during load driving is reduced.
FIGS. 26 and 27 are waveform diagrams showing signals according to respective positions of the driving circuit shown in FIGS. 22 to 25. The reference characters in FIGS. 26 and 27 correspond to the reference characters in FIGS. 22 and 25.
The load-driving state 5-a in FIG. 22 is represented in duration T6a in FIG. 26. Transistors 20 and 23 are turned ON and transistors 21 and 22 are turned OFF. The value of OUTP becomes VCC-level and the value of OUTN becomes GND-level. The electric-potential difference VL1 across the inductive load L1 becomes +VCC. Then, the current I flows through the inductive load L1 from OUTP to OUTN.
The load-driving state 5-b in FIG. 23 is represented in duration T6b in FIG. 27. Transistors 20 and 23 are turned OFF and transistors 21 and 22 are turned ON. The value of OUTP becomes GND-level and the value of OUTN becomes VCC-level. The electric-potential difference VL1 across the inductive load L1 becomes −VCC. Then, the current I flows through the inductive load L1 from OUTN to OUTP.
The load-driving state 5-c in FIG. 24 is represented in duration T6c of FIG. 26 or FIG. 27. Transistors 21 and 23 are turned ON and transistors 20 and 22 are turned OFF. The values of OUTP and OUTN become GND-level. The electric-potential difference VL1 across the inductive load L1 becomes zero (GND). Then, the current I flows through the inductive load L1 from OUTP or OUTN to the ground. That is, energy accumulated in the inductive load L1 is discharged.
The load-driving state 5-d in FIG. 25 is represented in duration T6d in FIG. 26 or FIG. 27. Transistors 21 and 23 are turned OFF and transistors 20 and 22 are turned ON. The values of OUTP and OUTN become VCC-level. The electric-potential difference VL1 across the inductive load L1 becomes zero (GND). Then, the current I flows from OUTN or OUTP to the power source. That is, energy accumulated in the inductive load L1 is discharged.
In other words, when the value of OUTP is higher than that of OUTN in the signal component excluding the switching frequency component and harmonic components applied across terminals of the inductive load L1, driving is performed in such a signal waveform, as a timing chart 6-a shown in FIG. 26, such that durations T6c and T6d that no energy is supplied exist before and after the duration T6a that a current as energy is supplied from OUTP to OUTN.
Similarly, when the value of OUTN is higher than that of OUTP in the signal component excluding the switching frequency component and harmonic components applied across terminals of the inductive load L1, driving is performed in such a signal waveform as a timing chart 6-b shown in FIG. 27, such that the durations T6c and T6d that no energy is supplied exist before and after the duration T6b that a current as energy is supplied from OUTN to OUTP.
However, when a piezoelectric speaker as a capacitive device is driven by use of the above driving method, a connection between load terminals is short-circuited for a duration that electric charge should be retained. Thus, the accumulated electric charge is lost, and the voltage across the load is lowered. In the next power-supply duration, another charge corresponding to the lost charge is additionally supplied in order to compensate for the lost charge, thus consuming electric power overly. That is, the above driving method produces reactive power and cannot obtain the advantage of low power consumption in a piezoelectric speaker.